North Korea doesn't have much of an economy, and few exports. Some of those exports, like illegal drugs and counterfeit currency, are under constant attack by other nations and international police. Ballistic missile exports are threatened by new sanctions. That leaves nuclear weapons technology, and expertise in building tunnels and bunkers. This last item has been a staple for over a decade. North Korea engineers have been seen at many underground construction projects. This has been observed in Iran, Syria and Myanmar (Burma). The North Koreans are very good at building things underground, and will help you, for a price, in cash or trade.
Going underground has long been used to protect military installations. The French built the Maginot Line in the 1930s, and the Nazis built factories and command bunkers underground during World War II. After World War II, the Soviet Union began building vast underground facilities (mainly to help survive a nuclear war.). It was during this Soviet construction effort that the North Koreans began digging.
North Korean skill at tunneling was first noticed during the 1950-1953 Korean War. The North Koreans, and their Chinese allies, were hard pressed by American airpower and artillery, and quickly discovered that the most reliable protection from observation, and the bombs, was by digging. Over the next half century, this led to a tunneling effort that resulted in over 8,000 underground facilities. A little over half of them are combat positions near the DMZ (Demilitarized Zone between North and South Korea ), where artillery and rocket launchers were kept, ready to be rolled out and fired at targets in South Korea. Other tunnels were used as headquarters, bases for military units and factories.
One of the few underground efforts that attracted much media attention were the tunnels dug beneath the DMZ. Between 1974 and 1990 four tunnels were detected by American and South Korea forces. Investigation revealed that these structures were up to 3,000 meters in length and were capable of infiltrating up to 30,000 troops an hour, including light armored vehicles and artillery. South Korean intelligence believes that up to 20 more tunnels may be lying dormant beneath the DMZ. Efforts to find these tunnels continues. Some North Koreas tunnels were dug along the shore, to shelter small combat ships and submarines. These tunnels are up to a kilometer long and 14-22 meters wide.
With this track record, the North Koreans peddle their tunneling services to whoever is willing to pay cash, be discreet and can keep secrets. Thus Iran provided North Korean tunneling expertise to Hezbollah in southern Lebanon. The Myanmar tunneling efforts, mostly under the newly constructed capital of Naypyidaw, 460 kilometers north of Rangoon, have largely been kept secret. That's not difficult to do, as Myanmar has been a police state for decades.
North Korea has thousands of experienced tunnel engineers and technicians. They still have plenty of work at home, but as long as there are tyrants living in fear, the North Korean tunnel experts will always have plenty of travel opportunities.